As for the mechanism
of a population
, one might look to E.O. Wilson
and other sociobiologists
who have proposed a very similar phenomenon.
However, with all matters of evolution (which is what would have developed such a mechanism), it can't be a conscious response nor a feeling : "oh, look there are more people, my child should be homosexual."
This naturally doesn't rule out a mechanism. What sociobiology has proposed, then, is that the increase in hormones from other members of the species penetrate the womb and effect the growing fetus. This, I think, sounds more plausible.
Fetal development is highly sensitive to hormones. In fact, hormones are what lead to differentiation of cells, formation of the body in general, and in specific, the brain, along I might add with almost every other aspect of the human being. An increase in some given hormore (it could likely be the sex hormone which alters the fetus) is precent in the environment and, therefore, in the womb, changing the development of the fetus. (Note: An increase of hormones in our food plays a similar role. We are still unsure of the effects of rBGH and similar agents which we daily consume.)
This explains why no genetic link has necessarily been found (which seems implausible anyway because of the reduction in offspring among homosexuals) and why smaller communinities/tribes would be less susceptible (smaller populations).
Furthermore, the evolutionary reason for the mechanism can also be postulated (and it also runs along the lines of common sense) : if a population which has a tendency to produce too many members also has a mechanism to reduce reproduction while simultaneously providing more members to care for the young (a possible evolutionary role for non-reproductive members of a society), then such a population would tend to not die out as easily.
Imagine two villages. Both are fairly well populated but on a limited amount of land/food. One population contains such a mechanism; the other doesn't. The first will naturally reduce reproductive rates with increased population; the other won't. And in cases of overpopulation, the food chain tends to break, causing a general collapse in the population. Such instability is not favored by evolution. Therefore, from the population with the mechanism more survive, allowing better chances for reproduction, and a larger population share. QED.